2A

• There are many membranes within cells, like those around organelles, and mainly the cell surface membrane, which controls which substances can enter or leave the cell. Membranes are also the site of many chemical reactions, flexible for shape change, and to bind vesicles.
• The membrane is composed of phospholipids, proteins, cholesterol and other components.
• The lipids in the membrane are polar lipids, with one end joined to a polar group, which is often a phosphate group. The two fatty acid chains are non-polar but the phosphate group has a negative charge, making the molecule polar.
• When phospholipids are in water, the heads dissolve because they are hydrophilic, but the tails are insoluble because they are hydrophobic. In order for the tails to avoid water, they can form a monolayer at the conjunction with air, or a micelle when surrounded with water, or a bilayer.
• A phospholipid is the basis of all membranes, but alone it only allows fat-soluble molecules to pass through. This is because polar molecules can't dissolve in lipids, so they enter the cell using proteins.
• The phospholipid bilayer can be described as a fluid mosaic model of the cell membrane. It is a fluid system, with other molecules fixed in place, and some floating in the structure. The more unsaturated fatty acids there are, the more fluid it is due to the kinks disrupting the regular pattern.
• Cholesterol - makes the bilayer more stable and strong, and is a barrier against ions
• proteins - have hydrophobic part in bilayer, with hydrophilic part on the outside of the membrane
• Investigations into bilayer:

  • 1890 - Charles Ernest Overton saw lipid-soluble substances passed easier
  • 1917 - Irving Langmuir found monolayer
  • 1925 - Gorter and Grendel discovered bilayer
  • 1935 - Dawson and Danielli made a model with proteins
• types of transport

  1. passive - takes place with concentration, pressure or electrochemical gradients, without energy
  2. active - using adenosine triphosphate which is produced during respiration
• There are 3 types of passive transport:

  1. diffusion
  2. facilitated diffusion
  3. osmosis
• There are 3 types of active transport:

  1. active transport
  2. endocytosis
  3. exocytosis
• diffusion - net movement of particles from an area of high concentration to an area low concentration, by the random movement of particles
• Diffusion depends on the kinetic energy of particles, but does not use metabolic energy. It works well for small molecules (e.g. oxygen, carbon dioxide) but it doesn't work for larger hydrophilic molecules or ions.
• facilitated diffusion - net movement of particles from an area of high concentration to an area of low concentration, through a channel protein, by the random movement of particles
• Large molecules and ions can't move through the membrane by simple diffusion, so they use specific proteins in facilitated diffusion. Channel proteins are open pores in the membrane that allow one specific type of molecule to pass through, depending on the molecules' shape and charge. Gated channels only open when a specific molecule is present, or if there is an electrical charge.
• osmosis - diffusion of water from an area of high water potential to an area of low water potential, down a water potential gradient, through a partially permeable membrane
• water potential - a measure of (kPa) of the potential for water to move out of a solution
• isotonic - water potential of the solution is the same as in the cell, so there is no net movement of water
• hypotonic - water potential of the solution is higher than in the cytoplasm of the cell, so water moves into the cell
• hypertonic - water potential of the solution is lower than in the cytoplasm of the cell, so water moves out of the cell
• Osmosis needs to be minimal in animal cells, because too much water causes the cell to burst (lysis) and too little water causes the cell to become flaccid.
• Plant cells are normally turgid. This happens when the force of the water moving in to the cell, because it is in a hypotonic solution, is balanced by the hydrostatic pressure, maintained by the cell wall, pushing the water out of the cell.
• Plant cells can become plasmolysed. This happens when cells are in a hypertonic solution, so water moves out of the cell, causing the cell membrane to pull away from the cell wall. Incipient plasmolysis is when half of the cells in a sample are plasmolysed.
• active transport - movement of molecules or ions across a cell membrane, against the concentration gradient, using energy in the form of ATP from respiration, using carrier proteins
• Active transport is a one-way system that is faster than diffusion.
• The carrier proteins, used in active transport, often span the whole membrane. They can be very specific and only move one type of molecule, or they can work for multiple similar molecules.

  1. molecule enters the carrier protein
  2. protein undergoes conformational change, using energy from ATP
  3. the shape changes pushes the molecule into the cell
• The energy needed for active transport comes from ATP, from the mitochondria. ATPases are enzymes that catalyse the hydrolysis of ATP, releasing energy.
• Endocytosis is used to bring large molecules into the cell, using vesicle formation. Vesicles are bags made of membrane to hold substances in the cell. Endocytosis infolds the membrane to form vesicles containing the substance, requiring energy from ATP. There are 2 types of endocytosis: phagocytosis for solids, and pinocytosis for lipids.
• Exocytosis is used to move large molecules out of the cell by the fusion of vesicles, containing the molecules, with the surface cell membrane, requiring energy from ATP.
• Single-celled and very small multicellular organisms have a large surface area to volume ratio so they can get oxygen for respiration through their outer body surface.
• Larger organisms have billions of cells in specialised tissues and organs. Substances need to travel long distances to get from the outer body surface to the cytoplasm of all cells, so diffusion is not fast enough.
• The metabolic rate of larger animals is higher than smaller organisms, which allows them to be active and control their body temperature. This means that demands of each cell are higher.
• Complex organisms have specialised systems for gas exchange, e.g. lungs, gills, leaves, tracheal system.
• Factors affecting gas exchange:

  • surface area - increased surface area increases diffusion
  • concentration gradient - maintaining steep gradients makes diffusion faster
  • thickness of exchange surface - shorter the distance, faster the diffusion
• Fick's law of diffusion:

  rate of reaction = surface area x concentration difference / thickness of exchange membrane
• Gas exchange features:

  • large surface area to compensate for sa:v ratio
  • thin layers from shorter diffusion distances
  • rich blood supply to maintain steep concentration gradient
  • moist surfaces for diffusion
  • permeable surfaces for free passage of gases
• All passages in human have a good blood supply, and they have mucus and hairs to remove dust, small particles, and pathogens from the air, which protects the lungs from damage and infection.
• nasal cavity - main route by which air enters the system, where air is warmed and moistened
• mouth - air can enter but it isn't cleaned, warmed or moistened